1. Field of the Invention
The present invention relates to an oxygen fuel (oxy-fuel) combustion system and more particularly to combustion system for fossil fuels that utilizes oxygen for combustion that includes closed loop control of the flame temperature in order to maximize the radiation heat transfer of the combustion thereby optimizing the efficiency of combustion system while maintaining the design aluminum melting or holding temperature or steam or process temperature and interior material temperatures.
2. Description of the Prior Art
Combustion systems for fossil fuels, i.e carbon based fuels, which utilize oxygen for combustion in lieu of air relatively well known in the art. Examples of such systems are disclosed in International Patent Application Publication No. WO 02/088400, assigned to the same assignee as the present invention, and U.S. Pat. No. 6,398,547. Such combustion systems have come to be known as oxy-fuel combustion systems and are used in various industrial processes including the production of steam in connection with electric power generating process and the melting of aluminum, for example. These oxy-fueled combustion systems have been developed to overcome various disadvantages of air-fueled combustion systems.
For example, a major disadvantage of air-fueled combustion systems is the production of green-house gasses, which are known to be harmful to the environment. In particular, air is comprised of about 79% nitrogen and 21% oxygen. In such air fueled combustion processes, NOx and other green-house gasses, such as carbon dioxide CO2 and sulfur dioxide SO2 are produced as a result of the combustion process.
Another disadvantage of such air-fueled combustion processes relates to fuel efficiency. In such air-fueled combustion processes, a considerable amount of fuel is expended to heat the nitrogen in the boiler or furnace. For example, considering a process for producing steam, enough heat must be created in the boiler by the combustion process to cause a sufficient amount of energy to be transferred to the water to cause the water to flash over to steam. Because of the vast amount of nitrogen in the boiler or furnace, a significant amount of heat and thus fuel are wasted heating the nitrogen in the boiler or furnace, which is processed and released as waste.
Such oxy-fuel combustion systems solve these problems and utilize relatively pure oxygen that is from about 85% to 99%+ pure. By using relatively pure oxygen, the amount of green house gasses is reduced and the efficiency of the fuel is significantly improved. Because of the relatively high cost of producing, such relatively pure oxygen, further attempts have been made to further increase the efficiency of said oxy-fuel combustion systems. For example, U.S. Pat. Nos. 5,545,031 and 5,575,637 disclose improved burners for use in oxy-fuel combustion systems which are configured to provide a relatively larger flame surface for improving the radiant heat transfer. The systems disclosed in the '031 and '637 patents do well to improve the radiant heat transfer in an oxy-fueled combustion process and thus improve the efficiency of the oxy-fuel combustion process. However, the improvements disclosed in these patents involve fixed variables in the combustion process, namely flame shape, and do not take into account other variables, such as the amount of fuel used in the combustion process.
As such, other developments have concentrated on controlling the amount of fuel supplied to the individual burners with the boiler or furnace for different stoichiometric ratios. More particularly, U.S. Pat. No. 6,398,547 discloses an oxy-fuel combustion system which includes a burner control system in which the fuel to each of the individual burners is periodically oscillated between a fuel lean mode and a fuel rich mode control over a relatively wide range of stoichiometric ratios. By oscillating the amount of fuel provided to the individual burners, the amount of excess fuel and oxygen in the boiler or furnace can be controlled to improve the efficiency of the combustion process.
Although the system disclosed in the '547 patent improves the efficiency of fuel usage in oxy-fuel combustion systems, the control system is fixed and is based upon a fixed waveform that is loaded into the system prior to the initiation of the combustion process. However, dynamic variables in the combustion process, such as flame temperature, are also known to affect the efficiency of the oxy-fuel combustion process. In particular, flame temperature is known to be a function of the amount of oxygen supplied to the burner. As set forth in the literature, for example, “Spectrometer-Based Combustion Monitoring for Flame Stoichiometry and Temperature Control”, by Carlos Romero, Xianchang Li, Shahla Keyvan and Rodney Rossow, Applied Thermal Engineering, Volume 25, Issues 5-6, April 2005, Pages 659-676, hereby incorporated by reference, “overall furnace (or boiler) stoichiometry control does not preclude inefficient operation of individual burners due to local maldistributions of air and fuel, or malfunction of burner hardware.” Thus, even though known oxy-fuel boilers and furnaces are known to be controlled from a stoichiometric standpoint to within ±5%, the flame temperature may vary during a combustion cycle for the reasons set forth above.
Thus, there is a need for a system that is responsive to changes in dynamic combustion variables, such as flame temperature, for improving the efficiency of the combustion process.